The present invention relates to two-wheeled vehicles and more particularly to a pair of tires of high transverse curvature, in particular for equipping motor-vehicles, to one of the tires forming said pair, the front one in particular, and to a method of controlling the vehicle behavior through said pair.
The invention is particularly directed to a pair of tires for two-wheeled vehicles in which the transverse curvature, at all events of a value not lower than 0.15, in the front tires is higher than in the corresponding rear tires.
Still more particularly, the invention is concerned with the structure of said pair of tires in other words, the invention relates to a pair of tires provided with a carcass of radial structure, preferably having a shape ratio (H/C)xe2x89xa680% and/or mounted on a mounting rim the width of which is xe2x89xa760% of the nominal cord of the tire.
It is known that when a two-wheeled vehicle takes a curvilinear trajectory, it tilts on its side internal to the bend forming an angle defined as xe2x80x9ccamberxe2x80x9d angle, the value of which can reach 65xc2x0 with respect to the vertical plane of the ground; due to this operation, tires give rise to a camber thrust counteracting the centrifugal force acting on the vehicle.
Originally, tires had a carcass structure comprising a pair of plies of rubberized fabric reinforced with cords symmetrically inclined to the equatorial plane of the tire, usually known as cross-ply carcass structure, and optionally a belting structure also made of pairs of rubberized fabric strips with cords angled to the equatorial plane of the tire.
This carcass structure was capable of developing important camber thrusts; in addition, the tire pair had a very homogeneous behavior on a bend in that although the two tires had different sizes, they exerted qualitatively similar camber thrusts which were largely sufficient to balance the centrifugal thrust acting on the vehicle.
Driving the vehicle on a bend therefore was not particularly difficult, in that the vehicle had neutral driving characteristics so that a driver could take a curvilinear trajectory almost istinctively, by merely tilting the vehicle, without making adjustments to the attitude thereof, in particular the steering angle of the handlebar.
In confirmation of the above, motorcyclists surely can remember that they could go along a bend without holding the handlebar with their hands, but by merely moving their own centre of gravity.
In contrast, problems existed in terms of ride comfort, stability, vehicle roadholding and driver fatigue, connected with an excessive stiffness of the tires. The tire structure, under the effect of an imposed deformation, stores up spring energy that is instantaneously returned when stresses stop, thereby amplifying disturbances transmitted from the road surface, which results in loss of stability on the part of the vehicle. In particular when running takes place on a straight road, this excessive stiffness causes oscillations of high frequency (8-10 Hz) on the front tire at low speed (shimmy effect) and oscillations of lower frequency (3-4 Hz) on the vehicle at high speed, so that driving becomes precarious.
In an attempt to obviate these problems, the use of radial-carcass tires having a belt structure made of textile or metallic cords has been recently introduced. In particular, the rear tire is provided with a belt structure comprising (sometimes in an exclusive manner) winding of cords preferably of metal, oriented in a circumferential direction, whereas the front tire has a belt structure provided with radially overlapped strips of inclined cords.
Surely this pair of tires has improved the situation in terms of ride comfort and driving stability. Oscillations of the vehicle at high speed on a straight stretch practically have disappeared, in that the rear tire has an important damping effect, while the shimmy effect substantially remained unchanged.
The behavior improvement on a straight stretch however has triggered a new problem consisting in that the radial structure of the tire, in combination with a belt of circumferentially disposed cords (at 0xc2x0) is unable to give rise to a camber thrust suitable for requirements, also taking into account the increasingly higher performance offered by vehicles.
More particularly, the rear tire furnishes a lower and qualitatively different thrust (that is of a linear type) than that (which is curvilinear) of the front tire. Consequently, the thus equipped vehicles have lost a neutral behavior and acquired an over-steering behavior. Thus, on a bend the rear wheel, unable to counteract the centrifugal force of increasing value acting on the axis, at a certain point skids, that is tends to come out of its trajectory moving to the outside of the bend whereas the front tire closes to the inside of the bend.
In other words, as the speed increases and the radius of curvature of the trajectory decreases, the vehicle inclination alone is no longer sufficient to compensate for the effect of the centrifugal force. Thus, the vehicle inclination is no longer sufficient for ensuring a running stability. Thus, increase in the thrust exerted by the tires is required and this increase is obtained by varying the vehicle attitude by an operation carried out by the driver by means of the handlebar, usually known to those skilled in the art as xe2x80x9cpush steerxe2x80x9d, that is by inclining the rolling plane of the front tire with respect to the tangential direction of the curvilinear trajectory through an angle, called xe2x80x9cslip anglexe2x80x9d directed in a direction opposite to the trajectory curvature.
Thus a full thrust is obtained which is the sum of a camber thrust resulting from the inclination of the equatorial plane of the tire relative to the vertical line and a slip camber caused by the angular variation of the rolling plane of the front wheel.
The value to be assigned to the slip angle depends on the structural and behavioral features of the front tire, that is on the link that the tire is capable of express between the slip angle value and the slip thrust value, in combination with its camber thrust and the thrust exerted by the rear tire.
On coming out of a bend, in contrast, an opposite operation is necessary, which is usually referred to by those skilled in the art as xe2x80x9cpull steerxe2x80x9d: i.e. the handlebar is to be inclined to the inside of the bend so as to lift the motor-vehicle up and follow the rectilinear trajectory again. As a result of the above, the vehicle behavior is greatly conditioned by the tire pair with which it is equipped, which pair must be therefore conveniently selected and verified.
Attempts to facilitate this task have already been patent EP 280,889 offers a method of carrying out this selection a prior without further experimental controls being required. In accordance with the teachings of this patent, if one defines as xcex41 and xcex42 the slip responses, that is those functions linking the slip angle with the slip thrust for the front and rear tires respectively, tire pairs for which function xcex42xe2x88x92xcex41 appears to be an increasing monotonic function of the vehicle shift angle will be acceptable.
Actually the method does not solve the problem because the slip response also depends on the inflating pressure of the tire and in any case the result does not have an absolute value. In fact, there were pairs of tires that came out to be unacceptable in spite of the fact that they complied with the stated rule and vice-versa.
In accordance with the invention the Applicant has now conceived that there was a different way of solving the problem arisen with these new tires. This way does not involve in accepting the existence of the problem and trying to select in the most efficient manner only those tires, among a great number of front and rear tires, that lend themselves to form an acceptable pair or combination for equipping the vehicle. Rather it involves planing the structure of the tire pair so as to avoid the arising of the problem itself, and thus give back a neutral behavior on a bend to the vehicle equipped with the tire pair of the invention, as well as the capability of absorbing shocks and damping vibrations.
In a first aspect, the invention relates to a method of controlling the overall slip thrust in a pair of tire mounted on a motor-vehicle, and more particularly a method of controlling the behavior of a motor-vehicle through the structure of its tires, and preferably a method of controlling the slip thrust exerted by the motor-vehicle along curvilinear trajectories by varying the thickness or density of the reinforcing cords of a layer of cords disposed circumferentially, at least in the belt structure of the front tire.
In a second aspect, the invention relates to a tire for two-wheeled vehicles comprising a radial carcass of toric form having a curvature ratio not lower than 0.3, provided with a belt comprising at least one radially outer layer of circumferential coils of reinforcing cords, distributed axially with a varying density from one end to the other of said belt.
In a further aspect the invention also relates to a pair of tires for two-wheeled vehicles, the tires comprising a radial carcass of toric form of high transverse curvature, having a central crown and two sidewalls terminating with a pair of beads for anchoring the tire to a corresponding mounting rim, a tread band applied crown-wise to said carcass and a circumferentially inextensible belt structure, interposed between said carcass and tread band, characterized in that:
the belt structure of the rear tire comprises at least one radially outer layer formed of a plurality of axially side-by-side circumferential coils of wound cords having a substantially zero angle with respect to the equatorial plane of the tire,
the belt structure of the front tire comprises at least one radially outer layer provided with a plurality of cord coils, disposed at a substantially zero angle with respect to the equatorial plane of the tire, axially distributed with a varying density from one end to the other of said belt, and preferably at least one additional reinforcing layer disposed at a radially inner position.
Preferably the tires of the pair have a curvature ratio not lower than 0.3, the rear tire having a curvature ratio lower than the front tire.
Conveniently, the radially outer layer of cords in the belt structure of the rear tire is formed of a single cord or a narrow band of rubberized fabric comprising 2 to 5 metal cords, of the high-elongation type, spiraled on said carcass, from one end to the other of said crown portion, according to an angle of a substantially zero value with respect to the equatorial plane of the tire.
As regards the front tire, conveniently the cord coils of the radially outer layer of the belt structure comprise metal cords of the high-elongation type, distributed with a progressively increasing density from the equatorial plane to the belt ends, being of a value not exceeding 8 cords/cm in a region of predetermined width disposed on either side of the equatorial plane.
Preferably, the cord coils of at least one of the layers are wound on another reinforcing layer in a radially inner position that, in a first embodiment, is a sheet of elastomeric material, located between the cord coils and the carcass ply, optionally filled with bonding means dispersed in the material.
Alternatively, in accordance with different versions, the radially inner layer, preferably in the belt structure of the front tire, can comprise two strips disposed axially in side by side relation, provided with reinforcing elements oriented in inclined directions in each strip and in opposition to each other in the two strips relative to the equatorial plane of the tire, or two radially overlapped strips, on each side of the equatorial plane, provided with reinforcing elements oriented in inclined directions in each strip and in opposition to each other in the two strips relative to the equatorial plane of the tire.
Conveniently, the reinforcing elements of tire radially inner layer are selected from the group comprising textile cords and metallic cords. In addition the reinforcing elements in one of the strips can be of a different material than that of the reinforcing elements of the adjacent strip in a radial direction.
As regards the radially inner layer consisting of a sheet of elastomeric material, the bonding means dispersed in the elastomeric matrix are preferably short aramid fibril-structure fibres homogeneously distributed in said elastomeric matrix with a density per unit volume included between 0.5% and 5% of the overall volume, and most preferably oriented along a preferential direction which is circumferential or inclined to the equatorial plane.
Advantageously, and in any embodiment, the radially inner layer can be broken at the equatorial plane of the tire over a portion of a width preferably included between 10% and 30% of the axial extension of the belt.